In recent years, communication devices such as mobile phones become widely used. There is a case where acoustic wave devices using an acoustic wave are used as filters of communication devices, duplexers and the like. As examples of acoustic wave devices, there are devices using Surface Acoustic Wave (SAW), devices using Bulk Acoustic Wave (BAW) and the like. Piezoelectric thin film resonators are devices which use BAW, and include an FBAR (Film Bulk Acoustic Resonator), an SMR (Solidly Mounted Resonator) and the like. In addition, there are devices using a Lamb Wave. When the electromechanical coupling factor of the piezoelectric film becomes large, frequency characteristics of the acoustic wave device are improved, and the bandwidth widening becomes possible.
Japanese Patent Application Publication No. 2002-344279 discloses a technique that improves characteristics of a piezoelectric thin film resonator by using the piezoelectric thin film including an alkali earth metal and a rare-earth metal. Japanese Patent Application Publication No. 2009-10926 discloses a technique that improves characteristics of the piezoelectric thin film resonator by using a piezoelectric thin film including scandium. IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS AND FREQUENCY CONTROL, vol.47, p.292, 2000 discloses a technique that increases the electromechanical coupling factor of the piezoelectric thin film resonator by controlling the orientation of the piezoelectric thin film made of aluminum nitride (AlN).
However, there may be a case that it is difficult to obtain the high-enough electromechanical coupling by conventional techniques.